Not applicable.
Not applicable.
The present invention relates to high-speed aircraft and space launch vehicle propulsion, specifically to a method of improving the performance and thrust level of turbojet engines when used in such vehicles and enabling turbojet engines to operate effectively at higher speeds and higher altitudes.
There are a number of valuable missions that can be performed by aircraft capable of operating both inside and outside the atmosphere. Such aircraft are often referred to as trans-atmospheric vehicles. They generally accelerate through the atmosphere using air-breathing engines, and, after reaching the limits of atmospheric flight, continue to accelerate outside the atmosphere using rocket engines till their final velocity is achieved.
Missions for such aircraft include high-speed long-range transports, military strike and reconnaissance aircraft, as well as orbital space transports. These extreme missions place severe demands on propulsion systems. They must deliver very high performance to efficiently achieve high velocities. They must also function from very low velocity during takeoff at sea level, to orbital velocities beyond the atmosphere.
Trans-atmospheric vehicles generally use a combination of air-breathing and rocket propulsion. air-breathing systems are valuable since they gather a significant fraction of their propellant from the atmosphere. This reduces the amount of propellant that must be stored onboard and increases overall vehicle efficiency. Consequently, air-breathing propulsion is often used to the greatest extent possible before exiting the atmosphere and accelerating to final velocity under rocket power.
Turbojet engines are attractive for such applications due to their high effective efficiency, as well as their operational flexibility. They are particularly valuable during takeoff and landing where their high efficiency at low speeds is critical. However, conventional turbojets are limited in their ability to operate at the high speeds and altitudes associated with trans-atmospheric flight. To extend the velocity and altitude that can be reached using air-breathing engines, a series of combined cycle approaches have been suggested. These cycles combine the positive attributes of turbojet engines with other air-breathing cycles, including ramjets and scramjets. Unfortunately, these combined cycle approaches are relatively heavy and complex.
These previous air-breathing concepts have been characterized by relatively low thrust-to-weight ratios. This is acceptable for missions where propellant economy during long periods of atmospheric cruise is important. However, trans-atmospheric and space launch missions are generally dominated by acceleration requirements where high thrust is often more advantageous than specific impulse. This is due to the increase in gravity and drag losses during extended acceleration periods. Consequently, an increase in engine thrust, even at relatively low specific impulse, can result in decreased overall propellant consumption since acceleration time decreases out of proportion to the increase in propellant flow.
To address the problem of low engine thrust-to-weight, several concepts have been proposed which utilize pre-cooling to densify inlet air. This increases the engine""s power density and allows it to operate at higher Mach numbers. These engines generally use liquefied hydrogen for fuel. Before entering the engine, the cold hydrogen is circulated through heat exchangers ahead of the turbojet inlet to cool the incoming air. This effectively cools the air, but also produces undesirable drag reducing overall efficiency. The heat exchanger is inherently heavy, and often accounts for more than 35% of the overall engine weight. It is also difficult to operate the heat exchange at low altitude because of the problem of ice formation in the heat exchanger.
What is needed is an air breathing engine which can provide increased thrust and operates over a wider flight envelope of Mach number and altitude.
The engine of this invention is a turbojet which allows improved thrust, reduced drag, higher operating velocities, and higher maximum altitude of operation. The engine has a duct with an inlet for admitting atmospheric air. Arrayed sequentially in the duct are an air inlet, liquid oxidizer injection nozzles, a compressor section for compressing the atmospheric air, a combustor section for heating the atmospheric air by combustion of fuel, a turbine for extracting power from the heated air, an afterburner chamber where additional fuel can be burnt with the exhaust of the turbine, a nozzle, and finally an expansion bell.
The engine can function as a normal jet engine, taking in atmospheric air, compressing the air in the compressor section, heating the air by the combustion of fuel, extracting power to operate the compressor section, with a turbine, allowing the turbine exhaust with or without additional heating by the combustion of further fuel to expand through the nozzle to generate thrust.
The engine has provisions for the injection of an oxidizer, such as liquid oxygen, upstream of the compressor section to cool and increase the oxygen content of the atmospheric air ingested by the compressor section. Cooling the incoming air reduces the air volume, which allows a fixed inlet to be matched to varying flow conditions, allows a greater mass of air to be ingested by the compressor section, and allows the compressor section to compress the incoming gases to higher pressure. Cooling of the incoming air also reduces the compressor outlet temperature. The compressed air with the added oxygen is heated by combustion of fuel and expanded through the turbine to provide power to drive the compressor section. The gases entering the afterburner chamber are at higher pressure, and have a higher oxygen content, and are heated with additional fuel at a stoichiometric mixture ratio, and are then directed through a nozzle and expansion bell to produce thrust.
The injection of liquid oxygen increases mass flow by increasing the effectiveness of the compressor section, and by the addition of the mass of the liquid oxygen. Increased mass flow and higher exhaust gas temperatures, due to additional fuel and higher combustion temperatures, result in higher thrust. The injected oxygen, by allowing the amount of air ingested by the compressor section to be increased, allows a fixed inlet area to be inlet matched to the varying mass flow with increased Mach Number. The addition of oxygen to the inlet air flow allows the engine to operate at higher altitudes by preventing flameout due to decreasing oxygen.
The engine thus can operate in the manner of a normal turbojet engine, or may be thrust augmented, and operated at higher altitudes by the addition of an oxidizer to the inlet air.
It is an object of the present invention to provide a jet engine of improved thrust.
It is another object of the present invention to provide a jet engine which can accomplish inlet matching, without a variable geometry inlet.
It is yet another object of the present invention to provide a turbojet engine that can operate at increased velocity before exceeding compressor temperature limits.
It is also an object of the present invention to provide a turbojet engine that can pre-cool its inlet air without the use of a heat exchanger.
It is an additional object of the present invention to provide a turbojet engine that can pre-cool its inlet air without the use of liquid hydrogen, allowing higher density hydrocarbon fuels to be used.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.